What causes garden pond water to go green

Green water in your garden pond is caused by an algae with the botanical name Chlamydomonas  (algae is actually a diverse group of aquatic organisms). Some are familiar to most of us like, seaweed, pond scum, green water, blanket weed or the algal blooms in lakes.

 

The two types of algae I'm going to show you how to get rid off, are;

  1. Green water (Chlamydomonas) 
  2. Blanket weed (Cladophora)
nature pond with blanket weed
nature pond with blanket weed
pond with green water
pond with green water

What problems do algae cause in a garden pond?

  1.  Green water algae is harmless to fish and other pond-life, but if not controlled and just left to multiply it will turn the pond water green, looking unpleasant and making it difficult to see your fish, newts, frogs and other pond-life and aquatic plants, spoiling the enjoyment of having a pond. Also, it can be embarrassing when showing of your pond. 
  2. Blanket weed if left unchecked can cause serious problems in a pond, depleting oxygen and is dangerous to fish and frogs, etc. trapping them in its slimy growth and it can also smother aquatic plants. In moderation, algae can provide great habitat for aquatic insects. 

The main cause for both green water algae and blanket weed is high fertility (to much food) from excessive nutrients, mainly  from fish poo (especially Koi who seem to spend all their time eating and pooing), uneaten fish food, organic debris (rotting pond plants and falling leaves), fertiliser run-off into the pond from higher ground and fertilisers added to plants you have bought from your garden center, insufficient shade in very sunny sites (algae thrives on sunshine). Some algae is natural in a newly built and filled pond and will clear if you follow the tips below.

An algae attack is normally worse in spring and summer, during sunny weather.

 

So what's the answer?

How to stop pond water going green without using chemicals?

The water in my nature pond is clear and apart from a few months after building and filling it with tap water, always has been clear. So what's my answer? It's simple, follow these good pond-keeping tips and don't use chemicals.

The algae that causes green water cannot be filtered out it is a micro-organism. Therefore, it must be starved out. 

Don't overstock with fish, avoid Koi completely in a wildlife pond, they eat everything in sight including the wildlife.
Never overfeed, once the pond culture is established there is no need to feed goldfish anyway. (If feeding the fish is part of the enjoyment you and the kids, or in my case grandchildren, get from having a pond in your garden follow the instructions on the packet and deduct about 30%. Don't overfeed.
Avoid letting fertilisers get into the water, from run-off in your garden or from pond plants you have bought to plant.  It is a good ideal to remove plants from pots, tip the contents of the pot onto flower beds or the compost heap and run the plant under a tap to rinse off any fertilised soil and plant into the substrate of the pond, covering the root with a rock to stop the plant floating off.
Remove excess rotting pond plants and fallen leaves, etc. I say excess, because in a nature pond some organic debris should be allowed to sink to the bottom to form the substrate on the bottom of the pond. This will be the basis for a stable pond environment. The micro-organisms which will take care of clear pond water need some subsoil to live in.
Cover at least half of the surface with floating or big leafed plants like water lilies. These plants compete with algae for light and so can make a big difference to reduce the levels of sunlight reaching the water.
Don't completely shade a nature pond all living things need light)

Oxygenating and aerating the water. I'm not going into a lot of technical stuff here, but a pond needs oxygen to live and also needs to release the gases produced by bacteria breaking down organic matter in the bottom of the pond.  Running water through a stream or over a waterfall with the pump placed at the end of the pond furthest away from the waterfall is the best way to release harmful gasses and ensure there is plenty of oxygen to be easily absorbed into the water. 
Oxygenating plants. In a new pond, stock Ceratophyllum demersum (Hornwort) at a rate of rate of three bunches per m2 and allow it to grow until about a third of your pond is filled, then in summer remove the yellowing stems leaving the dark green/olive brown stems. The bulk of Hornwort will drop down to the bottom of the pond in winter to protect itself from frost and ice. 
Never use chemicals to try to clear green pond water they can be expensive and cannot solve the underlying problem anyway so can't provide lasting protection. The underlying problems can only be solved long-term by following the good pond-keeping guidelines above.
You won't need this, but if you can't be bothered with good pond-keeping a UV unit combined with a biological filter system could help deal with green pond water. 

 

In Summary there's a variety of things to do and not to do that can help deal with green water, all aimed at starving out the algae that causes it. Mostly natural and easy to do without using chemicals, the methods include

  • avoiding fertiliser,
  • removing decaying matter,
  • not over-stocking with fish,
  • oxygenating and aerating the water,
  • avoiding chemicals, 
  • sensible shade planting and keeping a healthy garden.